The present invention relates to a method of cleaning flue gases from gaseous pollutants, formed during combustion of oil or the like, during start-up of a boiler for combustion of fossil fuel such as coal, biomass fuel or waste. According to the method, the flue gases are conducted from the boiler through a flue duct to a barrier filter, such as a bag filter, for separation of particulate pollutants.
In a known method, the flue gases formed during start-up of the boiler, from combustion of, for example, oil in an oil starter burner, are by passed the bag filter and are discharged directly into the atmosphere. The start-up proceeds until the boiler has reached the required temperature, that is, usually when the temperature has risen from an ambient temperature of about 20xc2x0 C. up to about 100-300xc2x0 C., whereupon the stationary combustion in the boiler of, for example, coal may be initiated, and the bag filter be put into operation.
The flue gases formed during the start-up of the boiler contain particulate pollutants, such as soot, and gaseous pollutants, such as hydrocarbon and sulphur oxides, as well as water steam. Since both the boiler and the subsequently located flue-gas cleaning system are cold, that is, assume ambient temperature, during the start-up, the greater part of the gaseous pollutants of the flue gases condense. The condensate and small droplets which are thus formed consist primarily of hydrocarbon, sulphuric acid (formed by sulphur oxides) and water. The cold apparatus parts of the flue-gas cleaning system substantially consist of it bag filter and flue ducts, and of an economiser (energy recovery stage) and an air preheater associated with the boiler. If the boiler has previously been in operation, which is normally the case, a considerable part of the pollutants emanate also from dust in the form of soot and fly ash, which have become deposited in the boiler and its channels during operation thereof. This dust loosens in connection with the start-up and accompanies the flue gases out into the atmosphere. Also emission of pollutants when switching to filter operation immediately after the stationary coal firing has been started usually occurs, since this switching takes a certain period of time. A start-up usually takes between 0.5 and 10 hours, so a considerable quantity of pollutants are discharged into the atmosphere before the bag filter is put into operation arid the coal firing has been started. During the start-up of, for example, a medium-sized power plant (300 MW), in addition to the above-mentioned gaseous pollutants, up to about 500 kg of dust may be emitted. These pollutants, of course, constitute a major environmental problem. The method also results in an unattractive plume of flue gases, coloured by the oil soot, leaving the smoke stack.
The requirements of the authorities on the emission level from, for example, plants where fossil fuels are burnt are directed both to the total emission quantity and to a maximum emission limit value. These requirements have lately become increasingly stricter in many countries, and for this reason it is often necessary nowadays to conduct the flue gases, formed during start-up of the boiler directly to the bag filter. However, this known method unfortunately results in problems for the filter bags of the bag filter. The filter bags are damaged by the condensate and the droplet which are formed by the gaseous pollutants of the flue gases and their water steam in that these clog and xe2x80x9ccementxe2x80x9d the filter bags during the start-up. This leads to a very rapid pressure drop across the filter bags, which necessitates a considerably more vigorous and frequent cleaning of the filter. After only about 10 start-ups, a filter bag may therefore be totally destroyed and must thus be replaced. This leads to a very short service life for the filter bags of the bag filter. In addition, the filter cages arranged in the filter bags are rapidly subjected to corrosion in this very moist environment caused by the condensate and the droplets. This corrosion is particularly manifest if a large number of start-ups have to be made during a short period of time. The extensive service and maintenance work which is caused thereby results in this method becoming very expensive.
In the light of what has been stated above, the object of the present invention is to achieve, during start-up of a boiler, for burning fossil fuel, biomass fuel or waste, a simple and inexpensive method for cleaning flue gases, whereby the effect of condensed gaseous pollutants and droplets is eliminated or at least considerably reduced.
The above object is achieved according to the present invention with a method of a kind described in the introductory part of the description and characterized in that finely-dispersed particles, preferably from fly ash, are introduced into and mixed with the flue gases in the flue duct upstream of and/or in the barrier filter, and that the finely-dispersed particles are separated in the barrier filter while forming a dust cake, whereby condensed gaseous pollutants and droplets, especially consisting of hydrocarbon, sulphuric acid and water, are taken up on the surface of the finely-dispersed particles in the flue gases and in the dust cake formed.
The basic inventive concept is thus that the finely-dispersed particles, supplied to the flue gases, are to take up the condensate and the droplets which are formed during start-up of a boiler during combustion of oil or the like, such that the textile filter is not damaged by clogging and/or xe2x80x9ccementingxe2x80x9d.
The finely-dispersed particles are suitably introduced in an amount which is in the interval of 1-500 g/Nm3, especially 5-50 g/Nm3, of flue gas. The particles particularly consist or fly ash and/or bottom ash from the boiler, but may also consist of other finely-dispersed particles which have corresponding properties of taking up condensed gaseous pollutants and droplets.
The finely-dispersed particles are preferably taken from an external storage container, such as a silo. In the event that fly ash is used, this may alternatively be taken directly from at least one of the dust hoppers associated with the barrier filter, whereas in the event that bottom ash from the boiler is used, this may alternatively be taken directly from the boiler.
According to an alternative embodiment of the invention, the finely-dispersed particles nay be introduced into and mixed with the flue gases upstream of an air preheater associated with the boiler.